1. Field of the Invention
This invention relates to the field of electrical switches and particularly to the field of remotely controlled electrical switches capable of interrupting the application of a source voltage to a load in response to an overload condition.
This invention relates more particularly to the field of solid state power controllers for dc (direct current) service or solid state electrical switches such as solid state relays for use in aircraft applications capable of detecting overload conditions, such as those caused by battle damage, and is able to instantly interrupt electrical service to the load automatically thereby preventing or limiting further damage to the service or load or lead wire. This invention circuit has the additional capability of being remotely resettable after clearing the damage or overload condition. Applications for the invention solid state dc switch include marine as well as aircaft and industrial applications that require noise free closure and interruption of dc source power, compact size, extended switch life, low power dissipation, immunity to high vibration, mechanical shock and high reliability.
2. Description of the Prior Art
Electromechanical switching devices using solenoid driven means to transfer eletromechanical contacts to apply a source voltage to a load, such as relays are well known. When coupled with an electromechanical circuit breaker, a relay provides a remotely controllable power control function capable of supplying ac or dc service to load via its closed contacts in series with an electromechanical circuit breaker. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure thereby affording low power dissipation.
As used in dc power control applications, mechanical contacts cannot apply or remove power to a load free of bounce and arcing. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.
The solid state dc SWITCH is known to overcome the problem of contact bounce in applying a source voltage to a load. Solid state dc switches are also able to interrupt load current. However, solid state dc switches introduce a substantial voltage drop at the point of circuit closure, thereby providing relatively high power dissipation in comparison to eletromechanical switching devices. Solid state dc switches find their greatest application in controlling electrical service to loads requiring relatively low load currents.
Circuit breakers used to protect the electrical service for a load are typically complex mechanical devices that rely on magnetic circuits or thermally actuated means to detect an overload condition. Environmental effects such as vibration, moisture, temperature and age along with operational effects such as thermal and mechanical fatigue from past overload cycle experiences contribute to the degradation of these assemblies. Contact arcing that is sustained by a high voltage source as the contacts part to protect a circuit experiencing overload conditions, contribute to circuit breaker contact degradation, and produce transient noise disturbances that can contribute to error logic signal generation. In addition, mechanical switches provide no means for controlling the rate of rise of voltage on the load as the switch is closed.